An uncompressed image (at common 32-bit colour depth) is just over 3.8MB per megapixel (MP).
Low-end phones usually have cameras rated at 8MP to 16MP. Top-end phones can have cameras rated at 48MP or 64MP or more. Cameras are a big feature so they keep improving their capacities.
An uncompressed 16MP image is about 61MB. Public Mobile (Telus) "3G" sends data at 3Mbps which is basically 366KB/s or 0.36MB/s. That's basically 5.3 seconds. An uncompressed 48MP image would take 16 seconds. And MMS protocols usually impose a timeout after 10-12 seconds ... an oversized MMS attachment will be slow, even glacial, as it gets broken down into manageable chunks, and it will waste time between each step before it even reports failure.
Set lower resolution on camera photos. And use any image compression the camera software supports. Or install a photo/video compression app which can transcale and transcode and (de)compress these things for you.
You might need maximum resolutions and lossless image quality to capture important details on important things. But you only need passable half-decent quick and convenient image settings for ordinary things.